1. Field of the Invention
This invention pertains to a combustion process and a device in which the fuel supply is provided by at least one burner equipped with at least one injector.
The invention will be described specifically for use in melting glass in glass-making ovens, particularly ovens used for making float-type flat glass or ovens used to make hollow glass containers, for example, ovens that operate opposite to the type of ovens that use regenerators (energy recovery devices). However, the invention is not necessarily limited to such applications.
2. Description of the Related Art
The majority of combustion processes of the aforementioned type, particularly those used in glass-making ovens, are confronted with problems of undesirable NO.sub.x emissions. No.sub.x emissions are harmful to humans and to the environment. Indeed, NO.sub.2 is an irritating gas that causes respiratory ailments. Additionally, in contact with the atmosphere, these gases can gradually form acid rain. Finally, they cause photochemical pollution since in combination with volatile organic compounds and solar radiation, the NO.sub.x gases are the basis for the formation of so-called tropospheric ozone which, in increased concentration at low altitude, becomes harmful for human beings, especially when it is very hot.
All these factors mean that the standards with respect to NO.sub.x emissions are becoming increasingly restrictive. Currently, because of said standards, oven manufacturers such as those that manufacture glass-making ovens are constantly concerned with limiting the maximum level of NO.sub.x emissions, preferably at a rate less than 500 mg/m.sup.3.
The parameters that influence the production of NO.sub.x gases are known. One such parameter is temperature; beyond 1300.degree. C., the emission of NO.sub.x gases increases exponentially with excess air, since the concentration of NO.sub.x gases depends on the square root of that of oxygen or even the concentration of N.sub.2.
Many techniques have been proposed to reduce NO.sub.x emissions. One involves causing a reducing agent to convert the NO.sub.x gases to nitrogen. This reducing agent can be ammonia, but this has disadvantages including difficulties with storage and handling of such a product. It is also possible to use a natural gas as a reducing agent, but this has detrimental effects on the fuel consumption rate of the oven and increases the CO.sub.2 emissions.
Therefore it is preferable, although not mandatory, to avoid this technique by adopting the so-called primary measures. These measures are called "primary" because one does not attempt to destroy the NO.sub.x gases that are already formed, as in the previously described technique. Rather, one tries to prevent their formation, for example at the flame level. Additionally, these measures are simpler to implement and, consequently, more economical. They do not have to completely substitute for the aforementioned technique but can advantageously complement it. These primary measurements in general amount to an indispensable precondition for reducing the consumption of reagents of the secondary measures.
One can categorize, in a non-limiting way, the existing measures in several categories:
A primary category consists of reducing the production of NO.sub.x gases via the so-called "reburning" technique by which one creates an air-deficient zone at the oven combustion chamber level. This technique has the disadvantages of increasing the temperature at the regenerator stack and of requiring a specific design of the regenerators and their stacks, especially in terms of airtightness and resistance to corrosion.
A second category consists of affecting the flame by reducing or preventing the formation of NO.sub.x gases at that level. To do this one can, for example, attempt to reduce the amount of excess combustion air. It is also possible to attempt to limit the temperature peaks by maintaining the flame length and to increase the volume of the flame front in order to reduce the average temperature within the flame. Such a solution is, for example, described in French patent application FR 96/08663 and international application PCT/FR/97 01244, which were filed on Jul. 11, 1996 and Jul. 9, 1997, respectively. The solution consists of a combustion process for melting glass in which the liquid fuel supply and the supply of the gas and air mixture are both brought about in such a way as to spread out periodically the liquid fuel/gas-air mixture contact and/or to increase the volume of this contact in order to reduce NO.sub.x emissions.